Opiates (derived from the opium poppy) or opiate-like (synthetic) drugs are widely used to alleviate moderate to severe pain. These drugs are classified together as opioids. Opioids derived from the opium poppy include morphine and codeine. Opiate-like or synthetic drugs include fentanyl, meperidine and methadone.
Opioids bind to specific receptor molecules. Distinct categories of opioid receptors have been identified which include mu, delta and kappa receptors. (W. Martin et al., J. Pharmacol. Exp. Ther. 197, 517 (1977)).
Opioids are useful for various kinds of pain management. In particular, opioids are used to alleviate postoperative pain and chronic pain, such as cancer and neuropathic pain, and pain during labor and delivery. However, opioid use has been linked to many dangerous side effects, such as tolerance or physical dependence, constipation, cardiac depression and respiratory depression.
One major concern with opioid use has been the transfer of opioids across the placenta to the fetus. Opioids may adversely affect the fetus by compromising the delivery of oxygen and substrates from the mother. The respiratory depressant effects of opioids may also decrease fetal oxygen availability.
Another concern with the currently available opioids is that they are too short-acting for labor pain. For example, fentanyl usually provides relief for only 60 to 90 minutes. Labor pain can last up to twelve hours.
In addition, due to the depression of cardiac and respiratory function seen with opioids, dangers exist when opioids are used before and during surgery when the patient has compromised cardio-respiratory functions. For example, respiratory depression is especially risky for individuals who have compromised respiratory systems, such as asthmatics and smokers
Opioids negatively affect motor function and are also associated with undesirable sedative effects. This is problematic for orthopedic and joint replacement surgeries, which require immediate post-operative motility.
Accordingly, there is a need for pain management treatments that do not depress cardiac or respiratory function and do not affect motor function.
Further, there is a need for new pain management treatments that do not cross the placental barrier or compromise the maternal respiratory and cardiac function. In addition, longer acting treatments for pain are needed.
Additionally, it would be beneficial to have a pain management treatment that will be effective in mammals that have developed a tolerance to opioids.